1. Field of the Invention
This invention relates to line density multiplication in the area of nanotechnology, such as the fabrication of semiconductor devices and nanoimprint templates.
2. Description of the Related Art
Current photolithography has reached fundamental printing limits. As a result, it is necessary to develop processes for line density multiplication. One process that is gaining recognition for use in DRAM and NAND flash manufacturing is sidewall spacer “line doubling”, sometimes also referred to as “line multiplication”, “frequency doubling”, “self-aligned double patterning (SADP)”, “sidewall image transfer” or “pitch-halving”. The process also has application in making imprint templates, which may be used for making bit-patterned-media (BPM) magnetic recording disks. For example, U.S. Pat. No. 7,758,981 B2, which is assigned to the same assignee as this application, describes a method using sidewall spacer line doubling to make an imprint template with generally radial lines.
The process uses sidewall spacers to create patterned hardmasks as a means of doubling the line density. The prior art process is illustrated in FIGS. 1A-1F. A layer of hardmask material is deposited on a substrate, and a layer of mandrel material (which may be a photoresist) is patterned into lines on the hardmask layer (FIG. 1A). A conformal layer of spacer material is deposited on the tops and sides of the mandrel lines and on the hardmask layer in the gaps between the mandrel lines (FIG. 1B). The spacer material is typically an inorganic material, typically oxides like Al2O3, and is typically formed by atomic layer deposition (ALD). ALD is a well-known thin film deposition process based on the sequential use of a gas phase chemical process, in which by repeatedly exposing gas phase chemicals known as the precursors to the growth surface and activating them at elevated temperature, a precisely controlled thin film is deposited in a conformal manner. The spacer material on the tops of the mandrel lines and in the gaps between the mandrel lines is then removed by anisotropic etching, leaving the mandrel lines with sidewalls of spacer material (FIG. 1C). The material of the mandrel lines is then removed, leaving lines of sidewall spacer material on the hardmask layer (FIG. 1D). The number of spacer lines in FIG. 1D is double the number of mandrel lines in FIG. 1A, and thus the pitch of the spacer lines is half the pitch of the mandrel lines, hence the terms “line doubling” and “pitch halving”. The spacer lines are then used as an etch mask to transfer the pattern into the hardmask (FIG. 1E) and the spacer lines are then removed, leaving a pattern of hardmask lines on the substrate (FIG. 1E).
A problem with the prior art method of line density multiplication by the use of sidewall spacers is that the sidewall spacers formed on the mandrel stripes are used as the final etch mask to etch the substrate. However, the mandrel stripes are often not precisely perpendicular to the substrate, resulting in tilted sidewall spacers and degraded etched substrates.
What is needed is a line doubling process that does not require mandrel stripes and sidewall spacers.